Federal officials have information suggesting that al Qaeda has people in the United States preparing to mount a large-scale terrorist attack this summer, sources familiar with the information said yesterday.

Attorney General John D. Ashcroft and FBI Director Robert S. Mueller III intend to hold a joint news conference this afternoon to discuss the threat and to ask Americans to watch for several suspected al Qaeda operatives who may be in the country, officials said.

The concerns are driven by intelligence deemed credible that was obtained about a month ago indicating an attack may be planned between now and Labor Day.

That information dovetails with other intelligence "chatter" suggesting that al Qaeda operatives are pleased with the change in government resulting from the March 11 terrorist bombings in Spain and may want to affect elections in the United States and other countries.

"They saw that an attack of that nature can have economic and political consequences and have some impact on the electoral process," said one federal official with access to counterterrorism intelligence.

Intelligence and law enforcement officials are trying to strengthen security at the presidential nominating conventions this summer in Boston and New York. They are also concerned about the possible targeting of other prominent events, starting with the World War II Memorial ceremony Saturday in the District, the G-8 Summit June 8-10 in Sea Island, Ga., and the Summer Olympic Games in August in Athens.

Federal officials have been discussing raising the national threat level between now and Jan. 21, 2005, the day after the presidential inauguration, although Homeland Security Department officials said yesterday no such announcement is scheduled.

The Justice Department and the FBI plan to ask for the public's help today in locating several suspected terrorist sympathizers, including some whose names have not been made public before. The bureau likely plans another public push to find Aafia Siddiqui, 32,, a Pakistani woman who has a doctorate in neurological science and has studied at MIT and Brandeis University in the Boston area, as well as in Houston.

The FBI also could seek help locating a man Siddiqui has been linked to, Adnan G. El Shukrijumah. He is a suspected al Qaeda member who spent time in Florida, and his name has come up in interrogations of captured al Qaeda lieutenant Khalid Sheik Mohammed.

In April, an FBI bulletin to law enforcement agencies warned of possible truck bombs. A source familiar with the government's threat discussions said yesterday truck bombs are a primary concern.

"I'm more worried than I was at Christmastime," said one senior U.S. intelligence official, comparing the "election threat" to the cancelling of specific airline flights around the holidays. He said the U.S. government is convinced there are still as yet unidentified al Qaeda operatives residing in the United States, waiting for the word to launch plots.

"They are here, and there are indications they are preparing" attacks, said the official, who government policy bars from being named.

Another FBI bulletin, issued last week, urged law enforcement officials to be on the alert for possible suicide bombers. Officials were urged to take note of people dressed in bulky jackets in warm weather, clothing smelling of chemicals or trailing electrical wires, and they warned that potential bombers may be dressed in uniform or even disguised as pregnant women.

Within the last three weeks, members of the House and Senate intelligence committees have received briefings from the CIA and FBI on what the CIA counterterrorism center has termed the "election threat." The members have asked the agencies for more specific, follow-up briefings, including an assessment of al Qaeda's presence in the United States, congressional sources said.

One counterterrorism official said al Qaeda still aims to carry off a attack that would kill large numbers of people, and is aiming at modes of transportation such as airlines and ships. Anything less than a spectacular attack, such as a simple suicide bombing, would appear weak to al Qaeda's financiers, according to the counterterrorism official.

President Bush has said two-thirds of al Qaeda's pre-Sept. 11, 2001, leadership has been killed or captured.But CIA Director George J. Tenet has said many established terrorist groups that previously did not work together are making a concerted effort to undermine the United States.